In general, a lithographic printing plate consists of an ink-receptive image area for receiving an ink in the printing process and a hydrophilic non-image area for receiving a fountain solution. Lithographic printing is a method utilizing the mutually repelling property of water and printing ink, where the ink-receptive image area and the hydrophilic non-image area of a lithographic printing plate are used as an ink-receiving area and a fountain solution-receiving area (a non-ink-receiving area), respectively, to make a difference in adherence of the ink on the surface of the lithographic printing plate and after depositing the ink only on the image area, the ink is transferred to a printing material such as paper, thereby performing the printing.
For manufacturing such a lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an ink-receptive photosensitive resin layer (photosensitive layer, image recording layer) has been conventionally used. Usually, a lithographic printing plate is obtained by performing plate making according to a method where a PS plate is exposed through an original such as lith film and while allowing the image recording layer in the portion working out to an image area to remain, the image recording layer in other unnecessary portions is dissolved and removed with an alkaline developer or an organic solvent to uncover the hydrophilic support surface and thereby form a non-image area.
On the other hand, in recent years, a digitization technology of electronically processing, storing and outputting image information by a computer has been widespread, and various new image outputting systems responding to the digitization technology have been put into practical use. Along with this, a computer-to-plate (CTP) technique is attracting attention, where digitized image information is carried on a highly converging radiant ray such as laser light and a lithographic printing plate precursor is scanned and exposed by this light to directly produce a lithographic printing plate without the intervention of a lith film. Accordingly, it is one of important technical challenges to obtain a lithographic printing plate precursor responsive to such a technology.
The plate-making process of a conventional lithographic printing plate precursor requires a step of dissolving and removing the unnecessary image recording layer with a developer or the like after exposure and in view of environment and safety, the problem to be solved includes processing with a developer closer to the neutral region or reduction in wastewater generation. Above all, the treatment of wastewater discharged along with wet processing is recently a great concern to the entire industrial world from the standpoint of consideration for global environment, and the demand to solve these problems is becoming stronger.
As described above, from both aspects of consideration for global environment and adaptation to space saving and low running cost, it is more strongly required than ever to decrease the alkali concentration of developer and simplify the processing step. However, as described earlier, the conventional development processing comprises three steps of development with an aqueous alkali solution having pH of 11 or more, washing of the alkali agent in a water washing bath, and treatment with a gum solution mainly composed of a hydrophilic resin. Therefore, the automatic developing machine itself requires a large space and furthermore, there remains unresolved the issue in terms of environment and running cost, such as problem of treatment of development wastewater, washing wastewater and gum wastewater.
In this connection, for example, Patent Document 1 has proposed a development method of performing the processing with a low-alkaline developer containing an alkali metal carbonate and an alkali metal hydrogencarbonate and having a pH of 8.5 to 11.5 and an electrical conductivity of 3 to 30 mS/cm, but the developer does not contain a water-soluble polymer compound and requires a water washing step and a gum solution treatment step, and the issue in terms of environment and running cost has not been resolved.
Also, as described below, in Patent Documents 2 to 4, a treatment with a developer containing a water-soluble polymer compound such as sugar is disclosed.
Patent Document 2 discloses development processing with a gum solution preferably at a pH of 3 to 9. However, this gum solution does not contain a base component and therefore, the polymer of the photosensitive layer must be made hydrophilic to enable development, incurring a problem that the press life is seriously impaired. In addition, the hydrophilic protection of the non-image area by the gum component is insufficient and with an increase in the number of printed matters, the non-image area tends to be deteriorated in the stain preventing property.
Patent Document 3 discloses a plate-making method using a developer containing cyclodextrin or a derivative thereof, and in Examples, a developer having a pH of 3 to 9 and containing cyclodextrin together with a hydroxypropyl etherified starch and gum arabic is used.
Patent Document 4 describes in Examples a plate-making method of performing development processing with a processing solution having a pH of 11.9 to 12.1 and containing a film-forming polymer, wherein the method does not include a gum treatment step. It is stated that the film-forming polymer is preferably, among others, a polyhydroxy compound, and a sugar alcohol such as sorbitol is described as the polyhydroxy compound.
However, the sorbitol is a relatively small compound having a molecular weight of 182 and a carbon number of 6 and is weak in the film-forming property or adsorbability to the support as compared with a polymer compound, and a printing plate obtained by such processing has poor scumming resistance due to desorption of the gum component during printing. Also, the plate is in a state of an alkali still adhering to the plate surface, giving rise to a problem in view of safety of the worker, and moreover, there is a problem that when a long time is passed after preparation of the printing plate until printing, the image area is gradually dissolved to incur reduction in the press life or ink receptivity.
Even by these techniques, it has been difficult to provide a development processing solution having a pH stable to aging over a wide pH region from alkaline to neutral and having excellent film-forming property and a development method ensuring that even by simple processing of one solution and one step requiring no water washing step, excellent developability and good dispersibility of development scum are achieved and the non-image area can be kept from staining.